Spinal implant configured for midline insertion and related instruments

ABSTRACT

The embodiments provide a spinal implant that is configured for midline insertion into a patient&#39;s intervertebral disc space. The spinal implant may include structural guidance features to facilitate the angular approach of fixation elements into the apertures. The spinal implant may also be a configured with a tactile or visual feedback response feature to allow the user to know when the fixation elements are fully seated within the apertures.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional No. 61/793,847,filed Mar. 15, 2013, and entitled “SPINAL IMPLANT CONFIGURED FOR MIDLINEINSERTION AND RELATED INSTRUMENTS,” which is herein incorporated byreference in its entirety.

FIELD

The present disclosure relates to orthopedic implants, and moreparticularly, to spinal implants that facilitate fusion of bone segmentsand associated methods. Even more particularly, the disclosure relatesto a spinal fusion implant configured for midline insertion, and relatedinstruments.

BACKGROUND

The integrity of the spine, including its subcomponents like thevertebral bodies and intervertebral discs that are well known structuralbody parts forming the spine, are key to a patient's health. These partsmay become crushed or damaged as a result of trauma or injury, ordamaged by disease (e.g., by tumor, autoimmune disease) or as a resultof wear over time or degeneration caused by the normal aging process.

In many instances, one or more damaged structural body parts can berepaired or replaced with a prosthesis or implant. For example, specificto the spine, one method of repair is to remove the damaged vertebra (inwhole or in part) and/or the damaged disc (in whole or in part) andreplace it with an implant or prosthesis. In some cases, it is necessaryto stabilize a weakened or damaged spinal region by reducing orinhibiting mobility in the area to avoid further progression of thedamage and/or to reduce or alleviate pain caused by the damage orinjury. In other cases, it is desirable to join together the damagedvertebrae and/or induce healing of the vertebrae. Accordingly, animplant or prosthesis may be configured to facilitate fusion between twoadjacent vertebrae. The implant or prosthesis may be placed withoutattachment means or fastened in position between adjacent structuralbody parts (e.g., adjacent vertebral bodies).

Typically, an implant or prosthesis is secured directly to a bonestructure by mechanical or biological means. One manner of spine repairinvolves attaching a fusion implant or prosthesis to adjacent vertebralbodies using a fixation element, such as a screw. Most implants andtheir attachment means are configured to provide an immediate, rigidfixation of the implant to the implantation site. Unfortunately, afterimplantation the implants tend to subside, or settle, into thesurrounding environment as the patient's weight is exerted upon theimplant. In some cases, this subsidence may cause the rigidly fixedattachment means to either loosen, dislodge or potentially damage one ormore of the vertebral bodies.

Several known surgical techniques can be used to implant a spinalprosthesis. The suitability of any particular technique may depend uponthe amount of access available to the implant site. For instance, asurgeon may elect a particular entry pathway depending on the size ofthe patient or the condition of the patient's spine such as where atumor, scar tissue, or other obstacle is present. Other times, it may bedesirable to minimize intrusion into the patient's musculature andassociated ligamentous tissue. In some patients who have had priorsurgeries, implants or fixation elements may have already been insertedinto the patient's spine, and as such, an implant introduction pathwaymay have to account for these prior existing conditions.

Thus, it is desirable to provide an implant that can be easily insertedin accordance with a specific pathway or approach. For example, incertain situations, it is desirable to provide a spinal implant that canbe inserted using a midline approach. In addition, it is desirable toprovide an implant and associated fixation elements that can account forsubsidence that occurs with the implant subsequent to implantation whilealso providing rigid fixation.

SUMMARY

The embodiments provide a spinal implant that is configured for midlineinsertion into a patient's intervertebral disc space. The spinal implantmay have a body including one or more apertures. The apertures areconfigured to receive fixation elements, such as bone screws and thelike. The fixation element may comprise one or more anti-backoutfeatures, such as a split ring. The spinal implant may includestructural guidance features to facilitate the angular approach of thefixation element into the apertures. The spinal implant may also be aconfigured with a tactile or visual feedback response feature to allowthe user to know when the fixation elements are fully seated within theapertures.

The present disclosure describes a spinal implant that is configured formidline insertion into a patient's intervertebral disc space. Inaccordance with one exemplary embodiment, a spinal implant is providedhaving an upper surface, a lower surface, an anterior portion, aposterior portion and one or more apertures within the anterior portionfor receiving at least one fixation element wherein the implant isconfigured for midline insertion. All or some of the apertures may beconfigured to permit a predetermined amount of nutation by a fixationelement, thus allowing the fixation element to toggle from one positionto another. The spinal implant may additionally include anti-migrationfeatures.

In another exemplary embodiment, a spinal implant comprises a body andone or more apertures. The body may comprise an upper surface, a lowersurface, an anterior portion, and a posterior portion, wherein the bodyis configured for midline insertion between vertebral bodies of apatient's spine. The one or more apertures may be provided within theanterior portion of the body and can receive at least one fixationelement. At least one of the apertures is configured with either atactile or visual feedback response feature to allow the user to knowwhen the at least one fixation element is fully seated. In oneembodiment, the at least one aperture comprises a countersink with acenter that is offset to the axis of the aperture.

In still another exemplary embodiment, a spinal implant comprises a bodyand one or more apertures. The body may comprise an upper surface, alower surface, an anterior portion, and a posterior portion, wherein thebody is configured for midline insertion between vertebral bodies of apatient's spine. The one or more apertures may be provided within theanterior portion of the body and can receive at least one fixationelement. At least one of the apertures is configured with a structuralguidance feature to facilitate approach of the at least one fixationelement into the opening. In one embodiment, the structural guidancefeature comprises a reverse chamfer over the at least one aperture.

In yet another exemplary embodiment, a method of treating a patient'sspine comprises accessing at least a portion of a patient's spine via aposterior, midline approach. A spinal implant is then inserted betweenvertebral bodies of the patient's spine, wherein the spinal implantcomprises a body having an upper surface, a lower surface, an anteriorportion, a posterior portion, wherein the body is configured for midlineinsertion between vertebral bodies of a patient's spine, the implantfurther including one or more apertures within the anterior portion ofthe body for receiving at least one fixation element. The spinal implantis attached with the at least one fixation element to the vertebralbodies and a predetermined amount of toggling of the fixation element ispermitted based on nutation of the fixation element during subsidence ofthe spinal implant.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are not restrictive of the disclosure. Additional features of thedisclosure will be set forth in part in the description which follows ormay be learned by practice of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, illustrate several embodiments of thedisclosure and together with the description, serve to explain theprinciples of the disclosure.

FIGS. 1A-1E show perspective views of an exemplary embodiment of aspinal implant of the present disclosure, in which:

FIG. 1A shows a perspective front view of the spinal implant;

FIG. 1B shows a top-down view of the spinal implant of FIG. 1 A;

FIG. 1C shows a perspective rear view of the spinal implant of FIG.

1A;

FIG. 1D shows a cross-sectional view of the spinal implant of FIG. 1A;and

FIG. 1E shows still another cross-sectional view of the spinal implantof FIG. 1A.

FIG. 2 illustrates a perspective side view of an exemplary embodiment ofa fixation screw of the present disclosure.

FIG. 3 illustrates an exploded view of an exemplary embodiment of ascrewdriver of the present disclosure.

FIG. 4A illustrates a top-down view of the inner shaft of thescrewdriver of FIG. 3.

FIG. 4B illustrates a side perspective view of the inner shaft of FIG.4A.

FIG. 4C illustrates an enlarged, exploded view of the universal joint ofthe screwdriver of FIG. 3.

FIG. 5A illustrates a cross-sectional view of the screwdriver of FIG. 3.

FIG. 5B illustrates a perspective view of a portion of the screwdriverof FIG. 3.

FIG. 6A illustrates a perspective view of an exemplary embodiment of anawl instrument with punch of the present disclosure.

FIG. 6B illustrates a cross-sectional view of the awl instrument andpunch of FIG. 6A.

FIGS. 7A-7C illustrate perspective views of various designs for thepunch of FIG. 6A.

FIG. 8 illustrates a perspective view of an exemplary embodiment of aspinal implant inserter of the present disclosure.

FIG. 9A illustrates an enlarged perspective view of a portion of theinserter of FIG. 8.

FIG. 9B illustrates a partial cutaway view of a portion of the inserterof FIG. 8.

DESCRIPTION OF THE EMBODIMENTS

The present disclosure describes a spinal implant that is configured formidline insertion into a patient's intervertebral disc space. Inaccordance with one exemplary embodiment, a spinal implant is providedhaving an upper surface, a lower surface, an anterior portion, aposterior portion and one or more apertures within the anterior portionfor receiving at least one fixation element wherein the implant isconfigured for midline insertion. All or some of the apertures may beconfigured to permit a predetermined amount of nutation by a fixationelement, thus allowing the fixation element to toggle from one positionto another. The spinal implant may additionally include anti-migrationfeatures.

Referring now to FIGS. 1A-1E, a spinal implant 10 of the presentdisclosure is shown. The spinal implant is configured for midlineinsertion into a patient's intervertebral disc space. The spinal implant10 may be employed in the lumbar or thoracic regions. Alternatively, thespinal implant 10 may be employed in the cervical region of the spine. Acervical version may be provided so long as it is appropriately sizedand configured, and the surgical approach takes into account thisspecific cervical design as well as size.

The spinal implant 10 may include anterior and posterior portions 12, 14and upper and lower surfaces 16, 18 profiled to correspond with theintervertebral space to which they are to be secured. The upper andlower surfaces 16, 18 may further include surface enhancements 28, suchas for example, teeth, ridges, protrusions, ribs, or fins, to enhancebone attachment, prevent migration and provide more stability. In oneembodiment, the enhancements 28 may be formed at about a 30 degree anglewith respect to the upper or lower surfaces 16, 18 of the implant 10. Inother embodiments, the enhancements can have an angle between about 25to about 35 degrees. It is understood, however, that alternative surfacemodifications, such as surface roughenings, barbs, spikes, bumps, etc.,may also be employed. Further, biological agents, such as bone growthfactors may be employed to enhance bone attachment, either alone or incombination with the mechanical enhancements described above.

In one embodiment, as shown in FIGS. 1C and 1D, the spinal implant 10defines a generally wedge shaped structure or arrowhead profile for easeof insertion and to be suitable for a posterior midline insertionapproach. As can be seen in FIGS. 1B and 1D, the implant 10 may haverounded edges. The anterior portion 12 extends into curved sidewalls 20that intersect with proximal portion 14 at posterolateral corners 22.The posterolateral corners may be rounded, as shown, to provide overallsmoothness to the implant profile and prevent undesirable damage tosurrounding tissue. The spinal implant 10, however, may have othershapes depending on the desired implantation site. Furthermore, edges ofthe implant 10 may be shaped so as to cooperate with insertion tools tominimize unintended distraction of the vertebral bodies between whichthe implant 10 is being positioned during implantation.

In one embodiment, projections 52 on the upper surface 16 of the implant10 may be provided to facilitate insertion. For instance, theseprojections 52 may be used to direct a tool or instrument. As shown inFIG. 1B, in one embodiment a pair of projections 52 a, 52 b may becentrally provided on the upper surface 16 of the implant 10. Theprojections 52 a, 52 b form a key way K for directing aninserter/distractor instrument's blade, and serves as a guide for suchinstruments.

The spinal implant 10 and its components may be formed of any suitablemedical grade material, such as biocompatible metals like stainlesssteel, titanium, titanium alloys, etc. or a medical grade plastic, suchas polyetheretherketone (PEEK) or another radiolucent material, ultrahigh molecular weight polyethylene (UHMWPE), etc. If so desired, theimplant 10 may also be formed of a bioresorbable material. Thebioresorbable material may be osteoconductive or osteoinductive (orboth).

As shown, the spinal implant 10 may include a central opening or lumen24 extending between the upper and lower surfaces 16, 18 to facilitatebony ingrowth or fusion between adjacent bone segments, such asvertebral bodies. If so desired, the opening 24 may be used to receiveand hold bone graft material, or other biologically active materialslike bone cement, bone void filler, bone substitute material, bonechips, demineralized bone matrix, and other similar materials. Thespinal implant 10 may be configured in a way that optimizes the opening24 such that the ratio of the cage or implant structure to the loadbearing area is as large as possible.

The spinal implant 10 may include holes 26 for placement of fixationscrews 60 therethrough to secure the spinal implant 10 to adjacent bonetissue. In the embodiment shown, the implant 10 includes three holes 26,such as one hole being centrally located (i.e., along the center line),and two laterally located (i.e., beside the center line.) Withoutcompromising stability, the lateral holes 26 should be located in amanner that avoids the need to retract vessels during surgery. It hasbeen postulated that extended retraction of vessels during surgery maylead to greater chances for complications to the patient. The lateralholes 26 should also be positioned so as to provide easier visibility ofthe surrounding implantation site for the surgeon. In the presentembodiment shown in FIG. 1B, the screw holes 26 are closely packed foreasier access around vessels.

FIG. 2 illustrates an exemplary fixation device such as a bone screw 60that may be used with the implants 10 of the present disclosure. Thebone screw 60 can have a head portion 62 and a tip 64 with a threadedshaft 68 in between. The bone screw 60 may also be used with ananti-backout ring 72. The screw 60 may also include a visual marker 66comprising a groove, band, laser etching, or other similar physicalindicator that disappears from view when the screw is fully seated, inorder to assist with the insertion process. For example, during use, agroove or band 66 laser marked on the screw head 62 disappears from viewwhen the screw 60 is fully seated within the screw hole 26 of theimplant 10.

As shown in greater detail in FIGS. 1A, 1B and 1E, the holes 26 may alsoinclude visual cues or markers to provide visual feedback to the surgeonthat a screw 60 inserted therein is properly seated. For example, in oneembodiment, the visual cue may comprise a groove 42 around the screwhole 26, or an indicator arrow 44 that can be seen when the screw 60 isseated fully. In one aspect of the embodiment, the visual groove 42 mayinclude an etching or a colored band. The visual groove 42 can beutilized to indicate that the screw 60 positioned therein is fullyseated or implanted, thereby allowing the screw head 62 to clear the wayfor the user to view the groove 42. Similarly, the indicator arrows 44may be utilized as visual checks by the user that the screws 60 arefully seated, as the arrows 44 would only be viewed upon fully seatingthe screws 60 after they have been placed into the screw holes 26.Accordingly, each of the screw holes 26 may be provided with one or moreof these visual markers (i.e., a visual groove 42 or an indicator arrow44, or both).

One skilled in the art will appreciate that the implant 10 may compriseany number of holes in any location on the implant 10. For instance, oneembodiment of the spinal implant 10 may employ two of holes 26 that arelocated on either side of the center of implant 10. Optionally, theimplant 10 may comprise other holes for receiving features like aradiologic marker or other imaging marker.

As shown in FIG. 1B, the spinal implant 10 may include bores 50 near theposterolateral corners 22 for receiving an imaging marker (not shown).The imaging marker may be formed of tantalum or a radiopaque material.The imaging marker may be configured as a rod or other appropriateshape. These imaging markers can assist with placement of the implant 10by providing visual cues for the surgeon intraoperatively. A suitableimaging marker is disclosed in co-owned U.S. Pat. No. 8,870,961,entitled “SPINAL IMPLANT CONFIGURED FOR MIDLINE INSERTION,” filed Nov.8, 2010, and issued Oct. 28,2014, the contents of which are incorporatedherein by reference.

The holes 26 provide a path through which securing means (e.g., fixationelements such as bone screws) may be inserted so as to secure theimplant 10 to respective superior and inferior vertebral bodies (notshown). The holes 26 may be configured to accommodate a variety ofsecuring mechanisms, such as screws, pins, staples, or any othersuitable fastening device.

The holes 26 of the spinal implant 10 may be configured to permit apredetermined amount of screw toggle (i.e., angular skew) and enable alag effect when the fixation screw is inserted and resides inside thehole or lumen 26. In other words, the holes 26 may be designed to permita certain degree of nutation by the screw, and thus, the screws maytoggle from one position to one or more different positions, forinstance, during subsidence. It also is believed that the predeterminedscrew toggle (permitted by the clearance between the lumen, or hole 26and the screw) promotes locking of the screw to the implant 10 aftersubsidence subsequent to implantation. In one embodiment, thepredetermined amount of screw toggle may be about 3 to 8 degrees, orabout 5 to 6 degrees.

As shown in detail in FIG. 1E, each of the holes 26 may have an openingwith a reverse chamfer or overhang feature. This overhang feature enablethe surgeon to better guide the insertion and general approach of thefixation screw 60 into the screw hole 26. In addition the apertures 26are configured not to break out onto the upper surface 16 of the implant10, and as shown in FIG. 1B the anterior portion 12 has a flat faceprofile to better match the vertebral anatomy and which contacts bonyendplates. As further shown in FIGS. 1A and 1B, the openings 26 may alsoinclude a relief 46, or cutaway portion, to promote visibility and easeof screw tip access. Moreover, the screw holes 26 may be provided on aflat face profile 54 of the implant 10, in order to better match thevertebral anatomy of the patient.

In one embodiment, the openings 26 may each include a countersink 40.The countersink feature's center is offset to the center axis of thehole 26, represented by lines C-C in FIG. 1E. This offset, representedby the arrows of FIG. 1E, allows a countervailing force when the surgeonapplies pressure on the fixation screw 60 during insertion, and providesa tactile feedback response to let the surgeon know when the fixationscrew's head 62 is properly seated. In other words, this offset causesthe screw head 62 to become loaded (i.e., provide feedback) on finalpositioning.

As further shown, a portion of the countersink 40 may have a sphericalsurface 30. The position of the spherical surface may be defined by thespherical radius center (represented by arrowed line SR). In otherembodiments, the openings or apertures 26 may be configured to provide avisual feedback response to the surgeon. Of course, the quality andstrength of the feedback response also depends on the quality of thebone tissue at the area of treatment. Healthy normal bone tissue willobviously provide the best feedback, as unhealthy, diseased or damagedbone tissue would not have sufficient strength to provide the necessarycountervailing force.

In one exemplary method of inserting the spinal implant 10, the surgeonprepares the implantation site by removing some disc material from thedisc space between two adjacent vertebrae. The spinal implant 10 may beprovided to the surgeon with the screws pre-attached, or separately, asdesired. Using a posterior midline approach, the surgeon then places theimplant 10 in the desired location of a patient's spine. Once in thecorrect location, the surgeon can tighten the screws into thesurrounding bone tissue, thereby securing the implant 10.

As noted, the implant 10 may be configured to permit a predeterminedamount of screw toggle and enable a lag effect when the fixation screwis inserted and resides inside the hole or lumen 26. Upon tightening,the lag effect may be observed whereby the implant 10 draws bone tissuetowards itself, which may promote better fusion.

As further noted, the predetermined screw toggle promotes locking of thescrew 60 to the implant 10 after subsidence subsequent to implantation.For example, after surgery, the patient's natural movement will resultin settling and subsidence of bone tissue and the implant 10 in situ. Itis believed that during this process, the weight exerted upon theimplant 10 causes the fixation screws 60 to toggle and consequently lockagainst one or more surfaces of the holes 26 of the implant 10.

Some practitioners prefer to allow some degree of movement between theimplant and the adjacent vertebral body after implantation. In that casethe screw heads may be provided with contours on its underside aspreviously discussed that allow the screws to nutate and toggle withrespect to the contoured holes 26 of the implant 10. Other practitionersmay prefer a more rigid implant that is firmly locked to the adjacentvertebral body. The embodiments of implant 10 allow either preference.

In a rigidly fixed version, the screws may be provided without thecontour on its underside (i.e., a relatively flat underside) while theopening 26 of the implant 10 would likewise not include a contoured seator countersink 40. Thus, when secured together, the screws and implant10 may form a rigidly locked construct. Where rigid fixation is desired(i.e., no toggle), the underside of the screws may also include surfacesfeatures as well in order to provide secure attachment to the implant10.

While a toggle and a rigidly fixed version of the implant 10 and screws60 are described, it is understood that a combination of toggling andrigid fixation may be accomplished in a single implant 10 and attachmentsystem. For example, it is possible to provide an implant 10 that allowstoggling of one or more screws 60, while also allowing rigid fixation ofthe other of the screws.

It will also be appreciated that the angular positioning of the variousholes, as described above, allows the present implant 10 to be of arelatively small size and therefore insertable from a midline approachwithin the intervertebral spaces of the spine. Thus, it will beappreciated that the angular positioning of the holes can assisteffective operation of the implant 10 and the ability to “stack”implants in adjacent multilevel procedures without the securing meansinterfering with each other. Such a feature can be of major significancein some situations and applications.

As further noted, the predetermined screw toggle promotes locking of thescrew to the implant 10 after subsidence subsequent to implantation. Forexample, after surgery, the patient's natural movement will result insettling and subsidence of bone tissue and the implant 10 in situ. It isbelieved that during this process, the weight exerted upon the implant10 causes the fixation screws to nutate and/or toggle and eventuallylock against one or more surfaces of the holes 26 of the implant 10.

The present disclosure also provides instruments that are useful forimplanting the spinal implant 10 and for practicing the methodspreviously described. FIG. 3 shows an exemplary embodiment of a fixedangle screwdriver 100 of the present disclosure, and the inner shaft110. FIGS. 4A and 4B show other perspective views of the inner shaft 110of the screwdriver 100. As shown, the screwdriver 100 may have an angledneck portion 102 that has a fixed angle. FIG. 4C shows an enlarged andexploded view of the universal joint assembly 114 used with the innershaft 110 within the screwdriver 100.

FIGS. 5A and 5B illustrate other views of the fixed angle screwdriver100 that may be use in the lumbar region during the implantationprocess. As shown, the neck 102 of the screwdriver 100 may also bepre-bent and rigidly fixed. The universal joint assembly 114 promotes amore narrow diameter construct, without compromising strength, therebyproviding the benefits of a single piece instrument that is minimallyinvasive. Moreover, the single piece instrument can be easily ported forcleanability.

FIGS. 6A and 6B illustrate exemplary embodiments of an angled guided awl120 with punch 122 of the present disclosure. The punch 122 may extendin a handle or knob 128 to push the punch through the awl instrument120, as shown in cross-section in FIG. 6B. The angled guided awl 120 cancomprise a resected window 126 at an elbow of the instrument 120 toreduce drag and improve cleanability, as shown in FIG. 6A. Further,various wire or punch patterns and shapes may be employed to lower theforce requirements for extension and retraction. These reduced surfacearea wires, or punches 122A, 122B, 122C, may have various configurationssuch as the ones shown in FIGS. 7A, 7B and 7C. The patterns of theexemplary punches/wires shown are configured to maintain flexibility andstrength while reducing drag within the awl instrument 120.

Both the fixed angle screwdriver 100 and angled guided awl instrument120 are configured to allow easy insertion of the implant 10 in theconfined intervertebral space being treated. The slim profile andangularity of the instruments helps the surgeon navigate around theanatomy to properly position the implant 10 in a minimally invasivemanner, without causing unneeded damage to the surrounding tissues.

FIG. 8 illustrates an exemplary embodiment of a spinal implant insertioninstrument 200 of the present disclosure. The instrument 200 maycomprise an elongate shaft 202 that extends between a gripping end 210and a handle end 220. As shown in detail in FIG. 9A, the handle end 220comprises a hand-controlled actuator or wheel 224 that controls movementat the gripping end 210. As FIG. 9B shows, the gripping end 210comprises at least one fixed arm 214 for grabbing an opening 26 of thespinal implant 10. A movable arm 218 is also provided, which allows theuser to securely hold the implant 10 during insertion. This movable arm218 may be controlled by the actuator wheel 224, which can be turnedleft or right to effect movement of the movable arm 218 up and down, toengage with the central opening or hole 26 of the implant 10. Oneadvantage of the actuator wheel 224 being positioned away from theterminal end of the handle end 220 is that the actuator wheel 224 is notin the way of the impaction end, which is the terminal end of the handleend 220.

Although the following discussion focuses on spinal implants orprostheses, it will be appreciated that many of the principles mayequally be applied to other structural body parts within a human oranimal body.

Other embodiments of the disclosure will be apparent to those skilled inthe art from consideration of the specification and practice of thedisclosure provided herein. It is intended that the specification andexamples be considered as exemplary only.

What is claimed is:
 1. A spinal implant comprising: a body having anupper surface, a lower surface, an anterior portion, and a posteriorportion, wherein the body is configured for midline insertion betweenvertebral bodies of a patient's spine; and one or more apertures withinthe anterior portion of the body for receiving at least one fixationelement, wherein at least one of the apertures is configured with atactile or visual feedback response feature to allow the user to knowwhen the at least one fixation element is fully seated; the tactilefeedback response feature comprising a countersink with a center that isoffset to an axis of the aperture, a portion of the countersink having aspherically shaped surface, and a position of the spherically shapedsurface being defined by the spherical radius center.
 2. The implant ofclaim 1, wherein the visual feedback response future comprises a visualmarker within the one or more apertures.
 3. The implant of claim 2,wherein the visual marker comprises a physical feature, laser etchedband, colored band, or indicator arrow.
 4. The implant of claim 1,further including a pair of projections on the upper surface of theimplant to provide a keyed entry for an instrument.